The present invention relates to a diffusion membrane for an enzyme-based sensor, a sensor comprising the diffusion membrane as well as a method of producing the enzyme-based sensor and the use of the enzyme-based sensor for the detection and/or determination of a substance, in particular an enzyme substrate, e.g., glucose.
Enzyme-based sensors are widely used to determine substances of interest in a qualitative as well as a quantitative manner in the blood and in other body liquids. Enzyme-based sensors are in particular used for the determination of enzyme substrates. In an enzyme-based sensor a so-called sensing reaction (sometimes also referred to in the art as a “transducer reaction”) occurs wherein a substance is converted under participation of at least one enzyme into another substance, which can be detected directly or indirectly. An example of such a sensing reaction is the enzyme catalyzed oxidation of glucose. Usually, this reaction uses oxygen as an electron acceptor. In the course of the reaction, glucose is converted into gluconolactone and the oxygen is converted into hydrogen peroxide. A sensing reaction either could measure the consumption of glucose and oxygen or the production of hydrogen peroxide or gluconolactone.
An enzyme-based sensor usually comprises several layers, among them an enzyme layer and a cover membrane or outer layer. This cover membrane is directly in contact with the sample and limits the diffusion of the substances necessary for the sensing reaction, especially the enzyme substrate or cosubstrate.
Enzyme-based sensors can be provided as electrochemical sensors or as optical sensors (optodes). The construction and function of a glucose optode is for example described in U.S. Pat. No. 6,107,083. The construction and function of an electrochemical glucose sensor is for example described in International Publication No. WO 99/30152.
Particularly, enzyme-based sensors which are used for the determination of glucose, lactate or creatinine are typically constructed with oxidoreductases and the detection is based on the oxygen consumption. In this case, the sensor comprises a cover membrane being a porous or at least a permeable polymer membrane, which controls the permeation of both the enzyme substrate and oxygen.
The currently available diffusion membranes for enzyme-based sensor application suffer from various disadvantages. According to one approach known in the state of the art, cover membranes for enzyme-based sensor applications are preformed membranes consisting of microporous structures from non-hydrating polymers like polycarbonate, polypropylene and polyesters. The porosity of such membranes is provided by physical means, e.g., by neutron or argon track etching. Glucose and oxygen permeate across such membranes predominantly in these pores filled with blood or other body liquids. One major disadvantage is that such membranes are preformed and not castable. A preformed membrane has to be attached to the enzyme layer. Very often the membranes are mechanically attached to the enzyme layer. Such mechanical attachments are expensive and technically complex. Further problems occur insofar as it is difficult to apply the membrane onto the underlying layer without producing air bubbles. Similar problems also occur when the membrane is for example glued onto an underlying layer.
Another approach known in the state of the art are castable cover membranes. Such cover membranes are generally formed by applying a solution of a polymer to an enzyme layer and by evaporating the solvent. Such membranes consist of polymer structures with hydrophilic and hydrophobic regions. Upon exposure to water, the hydrophilic region of the membranes absorb water, thus providing in the swelled structure a permeation path, e.g., for glucose. However, those membranes provide no defined porosity. In this approach the polymer itself has to provide the permeation, therefore not all polymers are suitable and thus the election of polymers is limited.
One disadvantage is that polymers, which are suitable for the use of castable membranes, are very often soluble only in aggressive or toxic solvents. Examples for this are cellulose acetate, which is soluble in DMSO and acetone, and PVC, which is soluble in tetrahydrofurane and cyclohexanone. This circumstance is relevant not only for safety reasons but also because the enzymes present in the enzyme layer may be destroyed by these solvents. Moreover, the effectiveness of such a membrane depends upon the dispersion of the hydrophilic domains within the hydrophobic matrix. Since it is difficult to achieve a homogenous dispersion in and during the production process of the membrane, the consequence is an inhomogeneous distribution of the hydrophilic domains. This results for example in a poor reproducibility.
Offenbacher et al. (U.S. Pat. No. 6,214,185 B1) describe a cover membrane with better coating reproducibility. The membrane is made of a PVC copolymer which allows a quite satisfying adjustment of the permeability due to the presence of a hydrophilic copolymer component. However, such a PVC cover membrane shows limitations for multiple measurements when used for a sensor based on the consumption of oxygen since the regeneration of the oxygen reservoir of the sensor is very slow.